modules: Add placeholder for CONFIG_RUST

In order to be able to ifdef based on `CONFIG_RUST`, the config must be
defined in the main repo (because the rust module is optional).  Add
this as a placeholder.  If the module is not availabe, the symbol will
be present, and always disabled.  The module will provide a full
definition for the Kconfig.

Signed-off-by: David Brown <david.brown@linaro.org>

modules/Kconfig

@@ -29,6 +29,7 @@ source "modules/Kconfig.open-amp"
 source "modules/Kconfig.picolibc"
 source "modules/Kconfig.nxp_s32"
 source "modules/Kconfig.renesas_fsp"
+source "modules/Kconfig.rust"
 source "modules/Kconfig.silabs"
 source "modules/Kconfig.simplelink"
 source "modules/Kconfig.sof"

modules/Kconfig.rust

@@ -0,0 +1,5 @@
+# Copyright (c) 2024 Linaro LTD
+# SPDX-License-Identifier: Apache-2.0
+
+config RUST
+	bool